Conventional carbureted engines used in motor vehicles, particularly those used in many motorcycles, generally do not have shut off valves to allow turning the fuel flow off when the motor vehicle is decelerating.
Real-world and simulated drive cycles representative of how motorcycles are used generally include a significant amount of time spent decelerating with the throttle closed on the carburetor. Such periods can include the use of so-called engine braking, in which the operator releases the throttle control and the throttle valve closes to prevent air from flowing into the combustion chamber(s) of the engine. Any fuel that is delivered to the combustion chamber(s) of the engine during periods in which the throttle is closed in this manner is generally wasted.